"Tardigrades are microscopic animals that live in moss and the muddy sand on beaches. They can survive high temperatures, freezing, and crushing pressures by drying themselves up into a little hard ball, called a tun. Stick a tun in water and [...] it will rehydrate and regenerate back into a tardigrade. [...] It may well be the most adorable genome on Earth."
The Lion's Mane Jellyfish is the largest jellyfish in the world. They have been swimming in arctic waters since before the dinosaurs (over 650 million years ago) and are among some of the oldest surviving species in the world.
The largest can come in at about 6 meters and has tentacles over 50 meters long.